Neutron collimation and detector shielding system and design method thereof

Abstract

The invention relates to a neutron collimation and detector shielding system and a design method thereof. The method comprises the following steps: inputting a neutron source item through MCNP; then three-dimensional modeling is carried out on the neutron collimator leading-out hole channel; performing neutron transmissivity analysis, adjusting internal structure parameters of the neutron collimator according to an analysis result, and updating the adjusted internal structure parameters to the established three-dimensional model to obtain an updated three-dimensional model; and performing neutron transmissivity analysis on the updated three-dimensional model, and performing repeated adjustment until neutron shielding meets requirements. After the material and the shape of the neutron collimator are determined, MCNP software is used for simulating intensity information of neutrons and gamma rays which are received by a detector and generated by targeting, and a layered shielding strategy is adopted for designing a shielding structure of the detector. According to the neutron collimator, the experiment accuracy and reliability of the neutron collimator can be enhanced, and by efficiently absorbing and shielding neutrons, it is ensured that experiment devices and personnel are prevented from potential hazards of neutron radiation.

Description

Technical Field

[0001] The present invention belongs to the technical field of nuclear detector design, and particularly relates to a neutron collimation and detector shielding system and a design method thereof. Background Art

[0002] When constructing a gamma-induced positron annihilation spectrometer based on the (n,γ) reaction, ensuring a high-quality thermal neutron source is the core prerequisite for the successful operation of the entire device and achieving the expected functions. The thermal neutron source is the key to driving the (n,γ) reaction, and its thermal neutron quality is related to the measurement accuracy and efficiency of the gamma spectrometer. The quality of the thermal neutron beam can be measured by the following indicators: the neutron fluence rate at the exit of the collimator, the divergence angle of the neutron beam, and the impurities in the neutron beam, including neutrons with undesired energies and the radiation background. According to the source of the background, it can be divided into two types: beam-related and unrelated. The beam-related background comes from the scattered neutrons and γ-rays generated by the beam neutrons on the sample, air, and surrounding materials, and the background should be as low as possible.

[0003] In addition, the effective shielding technology of the detector system is one of the key factors for the spectrometer to achieve high-precision measurement. The shielding design aims to minimize the interference of noise, background radiation, and non-target gamma rays in the external environment, so as to ensure that the detector can accurately capture and record the specific gamma ray signals generated by the (n,γ) reaction. The quality of the shielding effect directly affects the data quality and analysis accuracy of the gamma spectrometer, and is a technical point that cannot be ignored in improving the overall performance of the instrument.

[0004] The neutron collimator can ensure the collimation of the neutron beam, that is, the directionality and parallelism of the neutron beam. This is crucial for neutron experiments because the accuracy of the experimental results largely depends on the quality of the neutron beam. By adjusting the neutron collimator, the neutron beam can be made more concentrated and stable, thereby improving the accuracy and reliability of the experiment. The neutron collimator can effectively reduce the stray neutrons from around the experimental device or other non-target areas. These stray neutrons will interfere with the experimental results, increase the background noise, and reduce the signal-to-noise ratio of the data. The design of the neutron collimator can ensure that only neutrons from the target area can pass through, thereby effectively reducing the background noise and improving the clarity of the data. Neutrons have a certain degree of radioactivity. If the neutron beam is not effectively controlled, it may cause damage to the experimental equipment and personnel. The neutron collimator can effectively absorb and shield neutrons through its structural design and material selection, thereby protecting the experimental equipment and personnel from neutron radiation damage.

[0005] Neutrons are penetrating particles. If the neutrons emitted by a neutron source directly irradiate a detector, they will interact with the detector, resulting in a decrease in energy spectrum resolution and causing interference to the measurement. Therefore, placing a shielding body between the nuclear detector and the neutron source can effectively block the neutrons and prevent their direct irradiation of the detector, thereby protecting the performance of the detector and the measurement accuracy. In nuclear detection experiments, background radiation noise is an important interference factor. The shielding body can absorb and shield the radiation from around the experimental device or other non-target areas, including neutrons and other types of radiation, thereby reducing the background noise and increasing the signal-to-noise ratio of the data. This is crucial for improving the accuracy and reliability of the experiment. Summary of the Invention

[0006] The object of the present invention is to meet the needs of nuclear detection experiments, and to provide a design method and corresponding structure of a neutron collimation and detector shielding system for detection instruments, so as to enhance the accuracy and credibility of the experiment and ensure that experimental personnel are protected from the harm of neutron radiation.

[0007] To achieve the above object, the technical solution of the present invention is as follows:

[0008] A design method of a neutron collimation and detector shielding system includes the design of a neutron collimator and the design of a detector shielding structure. The design of the neutron collimator includes the following steps:

[0009] (1) Input the neutron source term through MCNP software;

[0010] (2) Conduct three-dimensional modeling of the neutron collimator's outlet channel and define the material of the neutron collimator;

[0011] (3) Conduct neutron transmittance analysis, adjust the internal structure parameters of the neutron collimator according to the analysis results, and update the three-dimensional model established in step (2) according to the adjusted internal structure parameters;

[0012] (4) Conduct neutron transmittance analysis on the updated three-dimensional model again. If the neutron shielding performance does not meet the requirements, return to step (3) to adjust the internal structure parameters of the neutron collimator and update the three-dimensional model, and repeatedly perform the adjustment operation until the neutron shielding performance meets the requirements.

[0013] Furthermore, in the specific implementation manner, for the design method of the neutron collimation and detector shielding system as described above, in step (2), establish a three-dimensional model of a neutron collimator with a tapered outlet channel. The inlet diameter of the tapered outlet channel is larger than the outlet diameter, and the outlet diameter is determined according to the diameter of the target.

[0014] Further, in the specific embodiments, for the design method of the neutron collimator and detector shielding system as described above, in step (3), by repeatedly adjusting the inlet diameter of the conical extraction channel of the neutron collimator, the neutron fluence rate is maximized on the premise that the neutron shielding performance meets the requirements.

[0015] Furthermore, for the design method of the neutron collimator and detector shielding system as described above, the formula for calculating the average neutron fluence rate at the outlet of the neutron collimator in step (3) is as follows:

[0016]

[0017] Where, is the average neutron fluence rate, I0 is the neutron emission rate per unit area of the surface source, θ is the maximum beam divergence angle, and K is the shape factor of the collimator;

[0018]

[0019] l0 is the distance from the intersection of the diagonal lines of the inlet and outlet of the conical extraction channel of the collimator to the outlet, D is the distance from the collimator to the source surface, and L is the length of the collimator.

[0020] Further, for the design method of the neutron collimator and detector shielding system as described above, the design of the detector shielding structure includes:

[0021] According to the determined material and shape of the neutron collimator, use MCNP software to simulate the intensity information of neutrons and gamma rays generated by the target received by the detector, and design the detector shielding structure using a layered shielding strategy. The outer layer of the shielding structure shields neutrons, and the inner layer shields gamma rays. The thicknesses of the outer and inner layers of the shielding structure are determined by simulation using MCNP software.

[0022] Furthermore, the outer layer of the shielding structure uses boron-containing polyethylene material, and the inner layer uses lead material.

[0023] The present invention further provides a neutron collimator and detector shielding system obtained according to the above design method, including a neutron collimator and a detector shielding structure. The neutron collimator includes two closely connected cylindrical parts, made of high-density polyethylene. The diameter of the cylinder on the inlet side of the collimator is smaller than that on the outlet side, and the height of the cylinder on the inlet side is greater than that on the outlet side. The extraction channel of the neutron collimator is a conical hole, and the outlet diameter of the conical extraction channel is determined according to the diameter of the target. By repeatedly adjusting the inlet diameter of the conical extraction channel, the neutron fluence rate is maximized on the premise that the neutron shielding performance meets the requirements.

[0024] Further, a cadmium layer is covered on the outside of the cylinder on the outlet side of the neutron collimator.

[0025] Furthermore, for the neutron collimator and detector shielding system as described above, the detector shielding structure is integrally cylindrical, including an outer shielding body and an inner shielding body arranged coaxially. The outer shielding body is made of boron-containing polyethylene material, and the inner shielding body is made of lead material.

[0026] Furthermore, in the specific embodiment, for the neutron collimator and detector shielding system as described above, the diameter of the cylinder on the side of the collimator entrance is 15 cm, and the height is 20 cm; the diameter of the cylinder on the side of the exit is 25 cm, and the height is 10 cm; the entrance diameter of the conical extraction channel is 10 cm, and the exit diameter is 1 cm; the thickness of the cadmium layer covering the outside of the cylinder on the side of the collimator exit is 0.3 cm.

[0027] The beneficial effects of the present invention are as follows: The design method of the neutron collimator and detector shielding system provided by the present invention can enhance the experimental accuracy and reliability of the neutron collimator, and ensure that the experimental device and personnel are protected from the potential hazards of neutron radiation by efficiently absorbing and shielding neutrons. At the same time, the design of the detector shielding significantly reduces the interference of background noise, thereby improving the ratio of the data signal to noise (signal-to-noise ratio) and optimizing the data quality. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 is the schematic diagram of the structural parameter design of the neutron collimator of the present invention;

[0029] Figure 2 is the schematic diagram of the structure of the neutron collimator of the specific embodiment of the present invention;

[0030] Figure 3 is the schematic diagram of the structure of the detector shielding of the specific embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0031] In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention, and are not used to limit the present invention.

[0032] The present invention provides a design method for a neutron collimator and detector shielding system, including the design of a neutron collimator and the design of a detector shielding structure. The design of the neutron collimator is used to adjust the material, shape and size of the neutron collimator, and the design of the detector shielding structure is used to adjust the material, shape and size of the detector shielding structure.

[0033] MCNP (Monte Carlo N Particle Transport Code) is a general software package developed by the Los Alamos National Laboratory in the United States based on the Monte Carlo method for calculating neutron, photon, electron, or coupled neutron / photon / electron transport problems in three-dimensional complex geometries, and also has the ability to calculate the eigenvalue problems of nuclear critical systems (including subcritical and supercritical systems). The design method of the present invention is implemented based on the MCNP software. Those skilled in the art should be clear that the MCNP software is only a tool for the development of the design method. Starting from the design idea of the present invention, it is not limited to implementing the technical solution of the present invention only using the MCNP software.

[0034] The design of the neutron collimator includes the following steps:

[0035] (1) Input the neutron source term through the MCNP software.

[0036] (2) Perform three-dimensional modeling on the outlet channels of the neutron collimator and define the materials of the neutron collimator.

[0037] Thermal neutrons are usually captured using materials with high absorption cross-sections for shielding. Fast neutrons need to be first slowed down to thermal neutrons and then absorbed by materials with high absorption cross-sections for shielding. According to this principle, high-density polyethylene is determined as the material for the collimator.

[0038] Under the condition of the same beam divergence angle in the inner cavity of the collimator, the average neutron fluence rate at the outlet of the conical outlet channel of the collimator is higher than that of the straight-tube neutron collimator, generally about 3 times. Therefore, the outlet channel of the collimator of the present invention is selected to be conical in shape. The inlet diameter of the conical outlet channel is larger than the outlet diameter, and the outlet diameter is determined according to the diameter of the target.

[0039] (3) Conduct neutron transmittance analysis, adjust the internal structure parameters of the neutron collimator according to the analysis results, and update the three-dimensional model established in step (2) according to the adjusted internal structure parameters;

[0040] (4) Conduct neutron transmittance analysis on the updated three-dimensional model again. If the neutron shielding performance does not meet the requirements, return to step (3) to adjust the internal structure parameters of the neutron collimator and update the three-dimensional model, and repeat the adjustment operation until the neutron shielding performance meets the requirements.

[0041] When using the MCNP software to simulate and analyze the neutron collimator, by continuously adjusting the inlet diameter of the conical outlet channel of the collimator, the maximization of neutron flux is achieved on the premise of ensuring the neutron shielding effect.

[0042] The neutron collimator structure designed by the present invention comprises two closely connected cylindrical parts, as Figure 2 shown. The main function of the two cylinders is to slow down (i.e., reduce the speed) the fast neutrons passing through them. The diameter of the cylinder on the entrance side of the collimator is smaller than that of the cylinder on the exit side, and the height of the cylinder on the entrance side is greater than that of the cylinder on the exit side. This structure enables the first cylinder (the cylinder on the entrance side of the collimator) to be completely inserted into the neutron passage during experimental operations. To prevent neutrons from escaping through the gap between the passage and the first cylinder, the second cylinder is designed with a larger diameter. Except for the core part also made of high-density polyethylene, the outer surface of the second cylinder is covered with a layer of cadmium. The role of cadmium is to absorb those particles that have been slowed down to thermal neutrons.

[0043] After determining the material and shape of the neutron collimator, the present invention uses MCNP software to simulate the intensity information of neutrons and gamma rays generated by target shooting received by the detector. This information is all noise for the detector. Considering that neutron capture will generate gamma rays, a hierarchical shielding strategy is adopted in the design of the detector shielding structure: first, shield the neutrons in the outer layer, and then shield the gamma rays in the inner layer. The thicknesses of the outer and inner layers of the shielding structure are determined by MCNP software simulation. This method can effectively reduce the noise to a relatively low level.

[0044] The detector shielding structure designed by the present invention is generally cylindrical, including an outer shielding body and an inner shielding body arranged coaxially. The outer shielding body is made of boron-containing polyethylene material, and the inner shielding body is made of lead material.

[0045] Embodiment

[0046] The design method of the neutron collimator provided by the present invention has the following design principles for the collimator lead-out channel:

[0047] As Figure 1 shown, the distance D from the collimator to the source surface, the length L of the collimator, the entrance diameter Φ0 ’ and the exit diameter Φ0 of the collimator. These four parameters determine the geometric characteristics of the neutron collimator. For the convenience of calculation, D, L, Φ0 and the distance l0 from the intersection of the diagonal lines of the entrance and exit of the conical lead-out channel of the collimator to the exit are selected as independent variables. The maximum beam divergence angle θ value can be expressed as

[0048]

[0049] The purpose of the design is to find appropriate D, L, Φ0 and l0 in order to obtain a high neutron fluence rate and a certain beam divergence angle. The neutron fluence rate at any point outside the collimator exit can be calculated by the following formula:

[0050]

[0051] In the formula, I0 is the neutron emission rate per unit area of the surface source, ds is the area element on the surface source, Z is the distance from this point to the surface source, and the integration range is the area of the source observed at this point. When Z is much larger than the linear dimension of the surface source, the following approximate formula for calculating the average neutron fluence rate at the exit can be derived:

[0052]

[0053] In the above formula, K is the shape factor of the collimator, which is reasonably defined in the present invention. It can be seen from the above formula that the average neutron fluence rate is proportional to the square of the beam divergence angle, and the influence of the D value on it is extremely small. When the exit channel of the collimator is a straight pipe, L = 2l0 and K = 1 / 4; when the exit channel of the collimator is an ultimate conical pipe, l0 = l0 ’ , K = 1, so the ratio of the collimator shape factors of the conical pipe and the straight pipe is between 1 and 4. That is to say, under the condition of the same beam divergence angle, the average neutron fluence rate of the collimator with a conical exit channel is 1 to 4 times that of the straight pipe type collimator.

[0054] L can be determined by calculating the neutron transmittance of MCNP. When the target diameter Φ = 1 cm, Φ0 can also be determined. By repeatedly modifying l0 (that is, continuously adjusting the collimator inlet diameter Φ0 ’ ), and calculating the neutron transmittance through the MCNP software, select l0 that meets the maximum neutron transmittance (corresponding to the maximum neutron fluence rate), so as to determine the corresponding collimator inlet diameter Φ0 ’ , and then the various parameters of the neutron collimator can be confirmed.

[0055] The structure of the neutron collimator designed by the above method includes two closely connected cylindrical parts, as Figure 2 shown. In this embodiment, the target diameter Φ = 1 cm, so the outlet diameter of the conical exit channel is 1 cm. The first cylinder has a diameter of 15 cm and a height of 20 cm, and is made of high-density polyethylene. The main function of this part is to slow down (i.e., reduce the speed) the fast neutrons passing through it. The diameter of the immediately following second cylinder is 25 cm, and the height is reduced to 10 cm. Except for the core part also made of high-density polyethylene, the outer surface of the second cylinder is covered with a 0.3 cm thick cadmium layer. The role of cadmium is to absorb those particles that have been slowed down to thermal neutrons. During the experimental operation, first insert the first cylinder completely into the neutron passing channel, and then place the experimental target material at the outlet of the channel. A conical cavity is designed inside the collimator, with an inlet diameter of 10 cm and gradually shrinking to a 1 cm diameter at the outlet, such a design aims to increase the neutron fluence rate.

[0056] After determining the material and shape of the neutron collimator, this embodiment uses MCNP software to simulate the intensity information of neutrons and gamma rays generated by target bombardment received by the detector. These information are all noise for the detector. Considering that neutron capture will generate gamma rays, a hierarchical shielding strategy is adopted in the design of the detector shielding structure: first, neutrons are shielded in the outer layer, and then gamma rays are shielded in the inner layer. The thicknesses of the outer and inner layers of the shielding structure are determined by MCNP software simulation. This method can effectively reduce the noise to a relatively low level. The outermost layer of the detector shielding is made of boron-containing polyethylene. Boron-containing polyethylene can slow down fast neutrons and also absorb thermal neutrons. After boron reacts with thermal neutrons, gamma rays will be generated, and gamma rays will also be generated when thermal neutrons react with the target. Gamma rays have strong penetrability. Therefore, a lead shielding is needed in the inner layer to shield gamma rays and prevent the detector from being interfered. After MCNP simulation, the thicknesses of the boron-containing polyethylene and lead shielding are determined.

[0057] As Figure 3 shown, the detector shielding structure designed in this embodiment is generally cylindrical, including an outer shielding body and an inner shielding body arranged coaxially. The outer shielding body and the inner shielding body are respectively cylindrical, and are closely sleeved together. The outer shielding body is made of boron-containing polyethylene material with a thickness of 5 cm, and the inner shielding body is made of lead material with a thickness of 30 cm. The diameter of the cylindrical cavity inside the inner shielding body is 15 cm for arranging the detector.

[0058] Those skilled in the art will readily conceive of other embodiments of the present invention after considering the specification and practicing the invention disclosed herein. This application is intended to cover any variations, uses, or adaptations of the present invention, which follow the general principles of the present invention and include the common general knowledge or conventional technical means in the technical field not disclosed by the present invention. Thus, if these variations, uses, and adaptations of the present invention fall within the scope of the claims of the present invention and their equivalent technologies, the present invention is also intended to include these modified and adaptable variations.

[0059] The above embodiments are only illustrative examples of the present invention. The present invention can also be implemented in other specific ways or other specific forms without departing from the gist or essential features of the present invention. Therefore, the described embodiments should be considered illustrative rather than restrictive in any aspect. The protection scope of the present invention should be defined by the claims, and any changes equivalent to the intention and scope of the claims should also be included within the scope of the present invention.

Claims

1. A method for designing a neutron collimation and detector shielding system, including the design of a neutron collimator and the design of a detector shielding structure, characterized in that: The design of the neutron collimator comprises the following steps: (1) Input neutron source terms through MCNP software; (2) Conduct three-dimensional modeling of the neutron collimator lead-out channel and define the material of the neutron collimator; (3) performing neutron transmittance analysis, adjusting the internal structural parameters of the neutron collimator according to the analysis results, and updating the three-dimensional model established in step (2) according to the adjusted internal structural parameters; (4) Performing neutron transmittance analysis on the updated three-dimensional model again. If the neutron shielding performance does not meet the requirements, returning to step (3) to adjust the internal structural parameters of the neutron collimator and updating the three-dimensional model, the adjustment operation is repeated until the neutron shielding performance meets the requirements.

2. The method for designing a neutron collimation and detector shielding system according to claim 1, characterized in that: In step (2), a three-dimensional model of a neutron collimator with a tapered lead-out channel is established, wherein the inlet diameter of the tapered lead-out channel is larger than the outlet diameter, and the outlet diameter is determined according to the diameter of the target.

3. The design method of the neutron collimation and detector shielding system according to claim 2, characterized in that: In step (3), the entrance diameter of the conical lead-out channel of the neutron collimator is repeatedly adjusted to maximize the neutron injection rate while ensuring that the neutron shielding performance meets the requirements.

4. The method for designing a neutron collimation and detector shielding system according to claim 3, characterized in that: The formula for calculating the average neutron flux rate at the neutron collimator exit in step (3) is as follows: in, is the average neutron fluence rate, I0 is the neutron emission rate per unit area of ​​the surface source, θ is the maximum beam angle, and K is the shape factor of the collimator; l0 is the distance from the intersection of the entrance and exit diagonals of the collimator cone lead-out channel to the exit, D is the distance from the collimator to the source surface, and L is the length of the collimator.

5. The method for designing a neutron collimation and detector shielding system according to claim 1, characterized in that: The design of the detector shielding structure includes: According to the determined material and shape of the neutron collimator, the MCNP software is used to simulate the intensity information of neutrons and gammas generated by the target received by the detector; A layered shielding strategy is adopted to design the detector shielding structure. The outer layer of the shielding structure shields neutrons, and the inner layer shields gamma rays. The thickness of the outer and inner layers of the shielding structure are determined by MCNP software simulation.

6. The method for designing a neutron collimation and detector shielding system according to claim 5, characterized in that: The outer layer of the shielding structure is made of boron-containing polyethylene material, and the inner layer is made of lead material.

7. A neutron collimation and detector shielding system obtained by the design method according to any one of claims 1 to 6, comprising a neutron collimator and a detector shielding structure, characterized in that: The neutron collimator comprises two closely connected cylindrical parts, which are made of high-density polyethylene. The diameter of the cylinder on the inlet side of the collimator is smaller than the diameter of the cylinder on the outlet side, and the height of the cylinder on the inlet side is greater than the height of the cylinder on the outlet side. The lead-out channel of the neutron collimator is a tapered hole, and the outlet diameter of the tapered lead-out channel is determined according to the diameter of the target. By repeatedly adjusting the inlet diameter of the tapered lead-out channel, the neutron injection rate is maximized while ensuring that the neutron shielding performance meets the requirements.

8. The neutron collimation and detector shielding system according to claim 7, characterized in that: The outside of the cylinder on the exit side of the neutron collimator is covered with a cadmium layer.

9. The neutron collimation and detector shielding system according to claim 7, characterized in that: The detector shielding structure is cylindrical as a whole, and comprises an outer shielding body and an inner shielding body which are coaxially arranged. The outer shielding body is made of boron-containing polyethylene material, and the inner shielding body is made of lead material.

10. The neutron collimation and detector shielding system according to claim 8, characterized in that: The diameter of the cylinder on the inlet side of the collimator is 15 cm and the height is 20 cm; the diameter of the cylinder on the outlet side is 25 cm and the height is 10 cm; the inlet diameter of the conical lead-out channel is 10 cm and the outlet diameter is 1 cm; the thickness of the cadmium layer covering the outside of the cylinder on the outlet side of the collimator is 0.3 cm.

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